On an ISA 2004 (including Service Pack 1) I have configured the cache size.After a reboot I noticed the following error :

Disk cache D:\urlcache\Dir1 failed to initialize.

Some errors were encountered when ISA Server restored specific data cache files. ISA Server will now attempt to recover these files. These errors may have occurred because there was not enough time to complete all necessary shutdown operations, when ISA Server was previously shut down. To avoid this in future, you can increase the value of the following registry key:

HKEY_LOCAL_MACHINE\System\CurrentControlSet\Control\WaitToKillServiceTimeout

Identify the reason for cache failure by examining previous recorded events, or the error code. The error code in the Data area of the event properties indicates the cause of the failure (internal code: 504.1312.3.0.1200.50).
Data:
Solution

The solution was to add the Everyone group to the URLCache folder. Set the right for the Everyone group to Full Control. In each urlcache folder a Dir1.cdat is created.

The Everyone user must have full access rights to build the Urlcache folder during Setup, or if you change the partition and the cache must be moved.

For more info : Microsoft  Knowledge Base http://support.microsoft.com/default.aspx?scid=kb;en-us;319877