A number of companies make use of storage devices that are not physically attached to the machine using them, such as storage area networks (SAN) and network-attached storage (NAS), as well as more exotic geographically dispersed solutions.
The question is whether MSMQ can be used on these and the official answer is no.
This doesn't necessarily mean it won't work, though. As long as these SAN/NAS/wherever solutions support the standard file I/O API that MSMQ uses (WriteFile, flushViewOfFile, etc) then MSMQ is happy. Otherwise MSMQ will see any anomaly, such as a file I/O failure, and will exit. There is no code in MSMQ to deal with disk failure. It is the customer's responsibility to provide reliable fault tolerant hardware.