Like many people I repartitioned my MacBook Pro 2015’s disk using Disk Utility in order to create space for the El Capitan public beta. Disk Utility lets you “live repartition”, which shrinks the existing partition while the operating system is up and running.
Unfortunately, yesterday the Yosemite partition pretty much self-destructed — I tried to run Activity Monitor but was told it was corrupted, which raised an eyebrow. When I tried to repair permissions using Disk Utility I saw hundreds of “Error 5 Input/Output” errors. A quick Googling was inconclusive but indicated that this could mean imminent disk failure.
Luckily I had a Time Machine backup so I wiped and restored (both the Yosemite and El Capitan partitions), and things are back to usual now. I subsequently verified/repaired the disk and erased the free space to test for read/write errors, and have seen nothing. So I don’t think it was a hardware fault (at least I hope not…)
My theories are that one or all of these issues might’ve been at play
(1) There’s an issue with the new PCIe-based flash multi-channel storage and Disk Utility hasn’t been updated, so repartitioning will fail
(2) El Capitan is still a beta so doesn’t play nice with other operating systems/partitions
(3) Both partitions were protected with File Vault and perhaps this played some kind of part in the corruption.
But the advice is clear: Be very careful repartitioning on a MacBook Pro 2015 with the new PCIe-based flash storage.
UPDATE 23 July 2015: Apple has released a SSD firmware update for the mid-2015 MacBook Pros, of which my Mac is one. Apple claims this fixes a bug that in “rare cases” causes data corruption. Hmmm… If your Mac is eligible for the fix you’ll find it in the Updates tab of App Store. Apple advises you backup your data before applying the update. Interestingly I ran disk benchmarks before and after applying the update. Afterwards, the SSD was marginally faster, although not to any appreciable degree because the SSD is already insanely quick.