Thanks, Arthur. I've downloaded etrecheck (thanks for the tip, nice little app!), and here is its report - bit of a long letter, I'm afraid -
Problem description:
Unstable installation of Dropbox after having upgraded my Macbook Pro (early 2008) to Yosemite from Mavericks. Dropbox worked fine with Mavericks (2+ years) but now keeps disappearing from the sidebar menu at top of screen and although I have the icon in my Apps folder, that will not open the app itself. I have a copy of my data folders on my drive, of course, which do update with my online Dropbox folders most of the time, but not always!
EtreCheck version: 2.0.11 (98)
Report generated 8 November 2014 16:25:56 GMT
Hardware Information: ℹ️
MacBook Pro (15-inch Early 2008) (Verified)
MacBook Pro - model: MacBookPro4,1
1 2.5 GHz Intel Core 2 Duo CPU: 2-core
4 GB RAM Upgradeable
BANK 0/DIMM0
2 GB DDR2 SDRAM 667 MHz ok
BANK 1/DIMM1
2 GB DDR2 SDRAM 667 MHz ok
Bluetooth: Old - Handoff/Airdrop2 not supported
Wireless: en1: 802.11 a/b/g/n
Video Information: ℹ️
GeForce 8600M GT - VRAM: 512 MB
Color LCD 1440 x 900
System Software: ℹ️
OS X 10.10 (14A389) - Uptime: 8:47:23
Disk Information: ℹ️
Hitachi HTS542525K9SA00 disk0 : (250.06 GB)
S.M.A.R.T. Status: Verified
EFI (disk0s1) <not mounted> : 210 MB
Recovery HD (disk0s3) <not mounted> [Recovery]: 650 MB
Macintosh HD (disk1) / [Startup]: 248.86 GB (110.41 GB free)
Encrypted AES-XTS Unlocked
Core Storage: disk0s2 249.20 GB Online
USB Information: ℹ️
Apple Inc. Built-in iSight
HP Officejet Pro 8500 A909a
Apple Inc. BRCM2046 Hub
Apple Inc. Bluetooth USB Host Controller
Apple, Inc. Apple Internal Keyboard / Trackpad
Apple Computer, Inc. IR Receiver
Gatekeeper: ℹ️
Mac App Store and identified developers
Kernel Extensions: ℹ️
/Applications/VMware Fusion.app
[not loaded] com.vmware.kext.vmci (90.4.18) Support
[not loaded] com.vmware.kext.vmioplug.10.1.24 (10.1.24) Support
[not loaded] com.vmware.kext.vmnet (0104.03.86) Support
[not loaded] com.vmware.kext.vmx86 (0104.03.86) Support
[not loaded] com.vmware.kext.vsockets (90.4.23) Support
/Library/Application Support/MacKeeper/AntiVirus.app
[not loaded] net.kromtech.kext.AVKauth (2.3.6 - SDK 10.8) Support
[not loaded] net.kromtech.kext.Firewall (2.3.6 - SDK 10.8) Support
/System/Library/Extensions
[not loaded] com.ZTE.driver.ZTEUSBCDCACMData (1.2.19) Support
[not loaded] com.ZTE.driver.ZTEUSBMassStorageFilter (1.2.19) Support
Launch Agents: ℹ️
[not loaded] com.adobe.AAM.Updater-1.0.plist Support
[running] com.docudesk.deskpdf.userdaemon.plist Support
[running] com.kodak.BonjourAgent.plist Support
[loaded] com.oracle.java.Java-Updater.plist Support
Launch Daemons: ℹ️
[loaded] com.adobe.fpsaud.plist Support
[loaded] com.microsoft.office.licensing.helper.plist Support
[loaded] com.oracle.java.Helper-Tool.plist Support
[loaded] com.oracle.java.JavaUpdateHelper.plist Support
[running] com.zeobit.MacKeeper.AntiVirus.plist Support
[loaded] fi.polar.libpolar.plist Support
[running] fi.polar.polard.plist Support
User Launch Agents: ℹ️
[loaded] com.adobe.AAM.Updater-1.0.plist Support
[loaded] com.adobe.ARM.[...].plist Support
[running] com.google.Chrome.framework.plist Support
[loaded] com.google.keystone.agent.plist Support
[loaded] com.kodak.KODAK AiO Annual Opt.plist Support
[loaded] com.kodak.KODAK AiO Firmware Updater.plist Support
[loaded] com.kodak.KODAK AiO Software Updater.plist Support
[running] com.zeobit.MacKeeper.Helper.plist Support
User Login Items: ℹ️
iTunesHelper Application (/Applications/iTunes.app/Contents/MacOS/iTunesHelper.app)
Skype UNKNOWN (missing value)
Dropbox ApplicationHidden (/Applications/Dropbox.app)
AdobeResourceSynchronizer Application (/Applications/Adobe Reader.app/Contents/Support/AdobeResourceSynchronizer.app)
VMware Fusion Start Menu Application (/Applications/VMware Fusion.app/Contents/Library/VMware Fusion Start Menu.app)
WebSync Application (/Applications/WebSync.app)
Internet Plug-ins: ℹ️
Flip4Mac WMV Plugin: Version: 3.2.0.16 - SDK 10.8 Support
FlashPlayer-10.6: Version: 15.0.0.189 - SDK 10.6 Support
Default Browser: Version: 600 - SDK 10.10
AdobePDFViewerNPAPI: Version: 11.0.09 - SDK 10.6 Support
AdobePDFViewer: Version: 11.0.09 - SDK 10.6 Support
Flash Player: Version: 15.0.0.189 - SDK 10.6 Support
QuickTime Plugin: Version: 7.7.3
SharePointBrowserPlugin: Version: 14.4.5 - SDK 10.6 Support
JavaAppletPlugin: Version: Java 7 Update 71 Check version
User Internet Plug-ins: ℹ️
Google Earth Web Plug-in: Version: 7.1 Support
3rd Party Preference Panes: ℹ️
Flash Player Support
Flip4Mac WMV Support
Java Support
Time Machine: ℹ️
Skip System Files: NO
Mobile backups: ON
Auto backup: YES
Volumes being backed up:
Macintosh HD: Disk size: 248.86 GB Disk used: 138.46 GB
Destinations:
BIPRA 1TB [Local]
Total size: 999.86 GB
Total number of backups: 4
Oldest backup: 2014-10-08 23:25:14 +0000
Last backup: 2014-10-28 22:20:36 +0000
Size of backup disk: Excellent
Backup size 999.86 GB > (Disk size 248.86 GB X 3)
Top Processes by CPU: ℹ️
9% WindowServer
0% AppleSpell
0% fontd
0% notifyd
0% Dropbox
Top Processes by Memory: ℹ️
236 MB AntiVirus
190 MB com.apple.WebKit.WebContent
168 MB Finder
146 MB softwareupdated
120 MB WindowServer
Virtual Memory Information: ℹ️
55 MB Free RAM
1.85 GB Active RAM
1.81 GB Inactive RAM
575 MB Wired RAM
2.79 GB Page-ins
475 KB Page-outs