Disk will not mount, but shows up on System Profiler My internal 250GB failed on me the other day, so I replaced it with a new SSD and HDD combo. I have the failed disk connected via USB/SATA, but it will not mount. However, it shows up under System Profiler.

The following is from System Profiler:

USB to S-ATA:


Capacity: 250.06 GB (250,059,347,968 bytes)
Removable Media:  Yes
Detachable Drive: Yes
BSD Name: disk4
Product ID:   0x0731
Vendor ID:   0x05e3  (Genesys Logic, Inc.)
Version: 90.94
Serial Number:   37007
Speed:   Up to 480 Mb/sec
Manufacturer:    Genesyslogic
Location ID: 0x24130000 / 4
Current Available (mA):  500
Current Required (mA):   192
Partition Map Type:  GPT (GUID Partition Table)
S.M.A.R.T. status:   Not Supported
Volumes:

disk4s1:

Capacity:  209.7 MB (209,715,200 bytes)
BSD Name:  disk4s1
Content:   EFI

Macintosh HD:

Capacity:  184.48 GB (184,484,454,400 bytes)
BSD Name:  disk4s2

Unknown:
Unknown:
By SSD and HDD combo do you mean something like a Seagate Momentus?

I'm really confused by you saying that the disk failed (how did it fail?) but then you have connected it by USB and you expect it to work?

The report you posted doesn't show a 

Content:    Apple_HFS


as you might expect to see under Volumes, so it appears to have been corrupted.

If data on this volume is vital and you don't mind spending money to get it back then unplug it right away and take it to a professional recovery service. 

If it would be just "nice to have back" then you could try using the First Aid tab of Disk Utility, and see if it can recover the partition(s).