@HEXDIY
Het werkt beter als ik inderdaad alles afsluit, opnieuw opstart en dan werk.
Maar nu was ik een backup aan het maken (van externe HD op dezelfde externe HD) en nu crashed mijn computer weer. Externe harde schijf dus niet goed afgesloten tijdens de backup en nu kan ik de bestanden niet meer benaderen (in Finder blijft scherm leeg als ik de externe HD wil zien en klokje rechtsonder blijft maar draaien..)
Ik heb een EtreCheck gedaan. Hieronder het rapport. Zegt mij weinig ...
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EtreCheck version: 2.9.10 (261)
Report generated 2016-03-18 11:52:34
Download EtreCheck from https://etrecheck.com
Runtime 2:05
Performance: Excellent
Click the [Support] links for help with non-Apple products.
Click the [Details] links for more information about that line.
Problem: Apps are crashing
Description:
Organizer is crashing when making a backup
Hardware Information: ⓘ
    MacBook Pro (Retina, 13-inch, Early 2015) 
    [Technical Specifications] - [User Guide] - [Warranty & Service]
    MacBook Pro - model: MacBookPro12,1
    1 2.7 GHz Intel Core i5 CPU: 2-core
    8 GB RAM Not upgradeable
        BANK 0/DIMM0
            4 GB DDR3 1867 MHz ok
        BANK 1/DIMM0
            4 GB DDR3 1867 MHz ok
    Bluetooth: Good - Handoff/Airdrop2 supported
    Wireless:  en0: 802.11 a/b/g/n/ac
    Battery: Health = Normal - Cycle count = 90
Video Information: ⓘ
    Intel Iris Graphics 6100
        Color LCD 2560 x 1600
System Software: ⓘ
    OS X Yosemite 10.10.5 (14F1605) - Time since boot: about 3 hours
Disk Information: ⓘ
    APPLE SSD SM0256G disk0 : (251 GB) (Solid State - TRIM: Yes)
        EFI (disk0s1) <not mounted> : 210 MB 
        Recovery HD (disk0s3) <not mounted>  [Recovery]: 650 MB 
        Macintosh HD (disk1) / : 249.78 GB (128.52 GB free)
            Encrypted AES-XTS Unlocked
            Core Storage: disk0s2 250.14 GB Online
USB Information: ⓘ
    Seagate BUP Slim WH 2 TB
        Seagate Backup Plus Drive 2 (disk2s1) /Volumes/Seagate Backup Plus Drive 2 : 2.00 TB (1.42 TB free)
    Apple Internal Memory Card Reader 
    Broadcom Corp. Bluetooth USB Host Controller 
Thunderbolt Information: ⓘ
    Apple Inc. thunderbolt_bus
Gatekeeper: ⓘ
    Mac App Store and identified developers
Kernel Extensions: ⓘ
        /Library/Extensions
    [loaded]    com.seagate.driver.PowSecDriverCore (5.2.7 - SDK 10.4 - 2016-01-29) [Support]
        /Library/Extensions/Seagate Storage Driver.kext/Contents/PlugIns
    [not loaded]    com.seagate.driver.PowSecLeafDriver_10_4 (5.2.7 - SDK 10.4 - 2015-07-17) [Support]
    [loaded]    com.seagate.driver.PowSecLeafDriver_10_5 (5.2.7 - SDK 10.5 - 2015-07-17) [Support]
    [not loaded]    com.seagate.driver.SeagateDriveIcons (5.2.7 - SDK 10.4 - 2015-07-17) [Support]
System Launch Agents: ⓘ
    [not loaded]    5 Apple tasks
    [loaded]    148 Apple tasks
    [running]    39 Apple tasks
    [killed]    20 Apple tasks
    20 processes killed due to insufficient RAM
System Launch Daemons: ⓘ
    [not loaded]    47 Apple tasks
    [loaded]    135 Apple tasks
    [running]    62 Apple tasks
    [killed]    17 Apple tasks
    17 processes killed due to insufficient RAM
Launch Agents: ⓘ
    [not loaded]    com.adobe.AAM.Updater-1.0.plist (2015-10-23) [Support]
Launch Daemons: ⓘ
    [loaded]    com.adobe.agsservice.plist (2015-10-23) [Support]
    [loaded]    com.adobe.fpsaud.plist (2016-03-08) [Support]
    [loaded]    com.microsoft.office.licensing.helper.plist (2014-02-26) [Support]
    [running]    com.seagate.TBDecorator.plist (2013-10-12) [Support]
    [loaded]    jp.co.canon.MasterInstaller.plist (2015-11-11) [Support]
User Launch Agents: ⓘ
    [loaded]    com.adobe.AAM.Updater-1.0.plist (2015-05-16) [Support]
    [running]    com.akamai.single-user-client.plist (2015-09-23) [Support]
    [loaded]    com.apple.FolderActions.folders.plist
    [loaded]    com.google.keystone.agent.plist (2016-03-06) [Support]
    [running]    com.spotify.webhelper.plist (2016-01-29) [Support]
User Login Items: ⓘ
    iTunesHelper    Programma  (/Applications/iTunes.app/Contents/MacOS/iTunesHelper.app)
    Google Drive    Programma  (/Applications/Google Drive.app)
Other Apps: ⓘ
    [running]    com.adobe.ElementsOrganizer13.72168
    [running]    com.google.GoogleDrive.92048
    [running]    com.microsoft.Excel.91764
    [running]    com.microsoft.autoupdate.fba.92616
    [running]    jp.co.canon.ij.scanutility2.CIJSUAgent
    [running]    org.mozilla.firefox.49448
    [loaded]    372 Apple tasks
    [running]    126 Apple tasks
    [killed]    41 Apple tasks
Internet Plug-ins: ⓘ
    AdobeAAMDetect: AdobeAAMDetect 1.0.0.0 - SDK 10.6 (2015-10-23) [Support]
    FlashPlayer-10.6: 21.0.0.182 - SDK 10.6 (2016-03-11) [Support]
    EPPEX Plugin: 10.0 (2014-09-16) [Support]
    QuickTime Plugin: 7.7.3 (2016-01-29)
    Flash Player: 21.0.0.182 - SDK 10.6 (2016-03-11) [Support]
    Default Browser: 600 - SDK 10.10 (2015-10-05)
    SharePointBrowserPlugin: 14.6.1 - SDK 10.6 (2016-03-08) [Support]
3rd Party Preference Panes: ⓘ
    Flash Player (2016-03-08) [Support]
    Paragon NTFS for Mac ® OS X (2015-06-26) [Support]
    Seagate Dashboard for Mac OSX (2015-09-01) [Support]
    Akamai NetSession Preferences (2015-09-23) [Support]
Time Machine: ⓘ
    Time Machine not configured!
Top Processes by CPU: ⓘ
        96%    Finder
        10%    launchd
         7%    WindowServer
         6%    kernel_task
         3%    Adobe Elements 13 Organizer
Top Processes by Memory: ⓘ
    955 MB    kernel_task
    664 MB    Finder
    442 MB    firefox
    295 MB    Adobe Elements 13 Organizer
    123 MB    com.apple.WebKit.WebContent(2)
Virtual Memory Information: ⓘ
    25 MB    Free RAM 
    7.97 GB    Used RAM (651 MB Cached)
    9.68 GB    Swap Used 
Diagnostics Information: ⓘ
    Mar 18, 2016, 11:38:56 AM    /Library/Logs/DiagnosticReports/Finder_2016-03-18-113856_[redacted].cpu_resource.diag [Details]
        /System/Library/CoreServices/Finder.app/Contents/MacOS/Finder
    Mar 18, 2016, 10:47:47 AM    /Library/Logs/DiagnosticReports/Finder_2016-03-18-104747_[redacted].cpu_resource.diag [Details]
    Mar 18, 2016, 09:57:17 AM    /Library/Logs/DiagnosticReports/Finder_2016-03-18-095717_[redacted].cpu_resource.diag [Details]
    Mar 18, 2016, 08:28:09 AM    /Library/Logs/DiagnosticReports/Finder_2016-03-18-082809_[redacted].cpu_resource.diag [Details]
    Mar 18, 2016, 08:25:35 AM    Self test - passed
    Mar 17, 2016, 06:32:09 PM    /Library/Logs/DiagnosticReports/Adobe Elements 13 Organizer_2016-03-17-183209_[redacted].cpu_resource.diag [Details]
        /Applications/Adobe Elements 13 Organizer.app/Contents/MacOS/Adobe Elements 13 Organizer
    Mar 17, 2016, 06:09:13 PM    /Library/Logs/DiagnosticReports/Finder_2016-03-17-180913_[redacted].cpu_resource.diag [Details]
    Mar 17, 2016, 06:04:14 PM    /Library/Logs/DiagnosticReports/Kernel_2016-03-17-180414_[redacted].panic [Details]
    Mar 17, 2016, 04:33:43 PM    /Library/Logs/DiagnosticReports/Adobe Elements 13 Organizer_2016-03-17-163343_[redacted].cpu_resource.diag [Details]
    Mar 17, 2016, 04:32:28 PM    ~/Library/Logs/DiagnosticReports/XPCKeychainSandboxCheck_2016-03-17-163228_[redacted].crash
        /System/Library/Frameworks/Security.framework/Versions/A/XPCServices/XPCKeychainSandboxCheck.xpc/Contents/MacOS/XPCKeychainSandboxCheck
    Mar 17, 2016, 04:19:03 PM    /Library/Logs/DiagnosticReports/Adobe Elements 13 Organizer_2016-03-17-161903_[redacted].hang
    Mar 17, 2016, 03:04:38 PM    /Library/Logs/DiagnosticReports/Adobe Elements 13 Organizer_2016-03-17-150438_[redacted].cpu_resource.diag [Details]
    Mar 17, 2016, 02:01:15 PM    ~/Library/Logs/DiagnosticReports/FinderSyncAPIExtension_2016-03-17-140115_[redacted].crash
        com.google.GoogleDrive.FinderSyncAPIExtension - /Applications/Google Drive.app/Contents/PlugIns/FinderSyncAPIExtension.appex/Contents/MacOS/FinderSyncAPIExtension
    Mar 17, 2016, 02:00:05 PM    /Library/Logs/DiagnosticReports/Kernel_2016-03-17-140005_[redacted].panic [Details]
    Mar 17, 2016, 10:45:46 AM    /Library/Logs/DiagnosticReports/Adobe Elements 13 Organizer_2016-03-17-104546_[redacted].hang
    Mar 16, 2016, 12:21:44 PM    /Library/Logs/DiagnosticReports/Finder_2016-03-16-122144_[redacted].cpu_resource.diag [Details]
    Mar 16, 2016, 11:20:47 AM    ~/Library/Logs/DiagnosticReports/Adobe Elements 13 Organizer_2016-03-16-112047_[redacted].crash
    Mar 16, 2016, 11:05:57 AM    /Library/Logs/DiagnosticReports/Adobe Elements 13 Organizer_2016-03-16-110557_[redacted].hang
    Mar 16, 2016, 10:35:06 AM    /Library/Logs/DiagnosticReports/Adobe Elements 13 Organizer_2016-03-16-103506_[redacted].hang