That sounds rather unlikely since your hastable is incorrect.
The Key can not start with a number.
Like this it works for me

[hashtable]$unassignedDisks = @{
    Disk1 = "naa.600507681081001BA8000000000006FC"
    Disk2 = "naa.600507681081001BA8000000000006F8"
    Disk3 = "naa.600507681081001BA8000000000006F9"
    Disk4 = "naa.600507681081001BA8000000000006FA"
    Disk5 = "naa.600507681081001BA8000000000006FB"
}
$UnassignedDisks.Disk1