"Lost" 54GB of Disk Space

Can anyone help me...My Mac HD shows on the system as having a capacity of 148GB, and available space of only 12GB (ie 136GB used). Yet when I add up the size of each folder/sub-folder - it only totals 82GB. I have also used the free utility 'OmniDiskSweeper' to check the utilisation of disk space - and it shows 82GB used. If anyone knows how I can 'get back' the lost 54GB - I'd be very appreciative...Thanks

First things first - have you recently done either:
1. a serious disk repair,
or
2. an Archive & Install of a fresh OS?

I had something similar happen when I migrated from my MacBook Pro to my new iMac.
For some reason the migration quit before it was finished. I had 94 Gig tied up that I was unable to access. The account name was there but showed no data.
I restarted the migration and this time it went successfully. Now I had two accounts. I could still not access the first aborted one.

I did a Disk-Map using Grand Perspective (free GrandPerspective) and it showed the area as one file.

I solved this by doing a clean reinstallation of the Mac OS.

Hope this helps a bit.

Good Luck