I have plenty of disk space!
Q When processing raw files in Photoshop, it brings up the message ‘Your Mac startup disc has no space for application memory’ and I have to force-quit. I don’t have anything else running at the same time and I have an iMac with 16GB RAM, a 256GB SSD that shows 100GB free, plus a 2TB HD. I have nothing in the trash and don’t understand why this is happening. Kev Rayner
A OS X uses the startup disk to hold data from your currently running applications that hasn’t been accessed for a while. This is a completely automated process and you can’t turn it off. The OS constantly swaps portions of data in and out of the disk in order to keep the things you are most likely to want in physical RAM, since this is much faster.
Unlike most other UNIX-based operating systems, OS X only uses the startup disk for this and will allow this virtual memory to grow as much as needed, to the physical limit of the drive. When OS X can’t expand the amount of data in virtual memory, it concludes that the drive must be full and throws up the warning message.
In your case we know the drive definitely isn’t full, so something else must be causing it to fail. Faulty disk permissions can sometimes do this, and it’s an easy fix, so repair permissions on the startup disk from Disk Utility. If this doesn’t help, you may have a fault in your physical RAM. I have seen cases where a single faulty bit on one of the RAM modules causes a virtual memory swap to fail, erroneously triggering this message. Order a replacement RAM module from www.crucial.com and swap each of the RAM sticks in your iMac in turn until the problem goes away.
Finally, it’s also worth checking your Photoshop plug-ins. A bug in one of these can cause the amount of memory Photoshop uses to steadily grow during your session, and this can make transient memory errors like this more likely. Try disabling your plugins one at a time in Photoshop.
Bizarrely, faulty RAM can sometimes look like a problem with your hard disk.