Q When checking my startup disk, Disk Utility reported a ‘bitmap error’. Is this serious, does it mean my hard disk is about to fail, could it cause loss of Internet connection, and is it time to replace this seven year-old iMac with a new one?
A The bitmap on a Mac Extended (HFS+) volume maps out which parts of that volume are being used for data storage. If that becomes corrupted, it may be repairable, depending on other stored information.
Errors can occur by chance, perhaps because a rogue application has crashed during a file write operation. It is sometimes a symptom of your hard disk beginning to fail. If your drive supports S.M.A.R.T. status checks, as shown in Disk Utility and third-party SMART tools, they are well worth checking, as they should give early warning of imminent failure.
However if this disk is as old as your iMac, it is very likely to pack up in the next year or so; some last little longer than the warranty before failing. Although unlikely, this error could corrupt any software on the volume, thus could possibly explain Internet connection problems, but those are more likely from a more conventional cause.
If you want to use your iMac for another year or two, replacement of the disk would be wise, but new iMacs are gorgeous and would be a big step forward, with their greatly improved performance.
Updated from the original, which was first published in MacUser volume 26 issue 16, 2010.