I recently upgraded the OS of a mid-2012 MacBook Pro to Mojave. Things were fine for a while, but now start-up takes forever, the OS is very sluggish, and I keep getting the message "Your disk is almost full". I assume the slowness is a result of the full disk.
I deleted a bunch of large files (tens of GB worth), but the available space never increased. Oddly, the Storage dialog ("About this Mac" > "Storage") shows that "System" is taking up nearly 273 GB of space (see the gray block in the screenshot below).
I've searched all over and I can't find where those "System" files are.
Is this a Mojave bug? Does anyone know where to find those files, so I can clean them up? Is this a Time Machine issue (automatic back-ups are disabled, by the way)?
Thank you in advance for all of your suggestions.
P.S. I've already done a disk repair, rebooted several times, etc.
