From Support:
I have an update for this. For an installation, you have to extend the existing volume. The additional disk is usable for data storage, but not for installation.
list /sys disk all-properties
sys disk logical-disk HD1 {
mode mixed <------- this can be used to install
size 97280
vg-free 42160
vg-in-use 53868
vg-reserved 0
}
sys disk logical-disk HD2 {
mode none <---------- This needs to be 'mixed'
size 97280
vg-free 0
vg-in-use 0
vg-reserved 19456
}
Basically HD2 exists only for some objects (datastore) but is not a valid OS install location.
https://support.f5.com/csp/article/K16103 - Extending disk space on BIG-IQ Virtual Edition