Thanks for the reply JW. Help is appreciated.
I've tried looking into what you suggest but I'm unable to find the info in the commcell console. The exchange backups are running from a 32bit second commvault instance (srv-commvault_32) with the Exchange iDA so this is all occurring on the commvault server. I checked Properties on srv-commvault_32 (Content Indexing is unchecked), then on the Exchange Mailbox iDA, on defaultBackupSet, and on the S-T subclient but could not find an index cache setting anywhere. Based on all the errors it's D:\CV_IndexCache\CV_Index. D: has 68GB free of 293GB. There's a potentially related error in event viewer:
Program = CVD, Computer = srv-commvault
[D:\CV_IndexCache] does not have required minimum space of  MB. The space available is  MB.
I also tried manually starting a Full Backup of the S-T subclient but there's no option I can see for "Create New Index". Our commvault admin is away hence my unfamiliarity with the system.
It's strange that only one of the subclients is failing and it's not even the biggest of them. We're using 9.0.0R2 with SP10 btw.