Solaris 8 will support the bigger drives - Solaris 2.5.1 may not. If you want to use the 18GB drive, you would have to upgrade your OS (and even that isn't a sure thing - according to documentation you might not be able to use 2.5.1.- it matters what the 18GB drive is in )

From Sunsolve:
DISK ARRAY Minimum OS Release
==============================================================================
StorEdge A1000 2.5.1 HW 8/97 or later
StorEdge A3500 2.5.1[Note 8] + RM 6.1.1 Update 1 or later
StorEdge A3500FC 2.6 or later
StorEdge A5x00 2.5.1 HW 11/97[Note 9] or later
StorEdge D1000 2.5.1 HW 8/97[Note 10] or later
StorEdge T3/T3+ 2.6 or later + minimum set of patches (see T3 Software/Firmware Configuration matrix)
StorEdge 3310 8
StorEdge 3900/6900 8 10/01
StorEdge 9910/9960 2.6
StorEdge 9970/9980 Depends on the server. See Sun StorEdge 9970 and 9980 Systems: What Works With What
StorEdge D2 8 Plus recommended patch cluster
StorEdge S1 2.6

Also see Sunsolve - Sparc 5 hardware - a 18GB drive will not go inside.