What it is
A managed disk in the Unattached state, left behind after the VM it belonged to was deleted or resized. It stores nothing anyone uses, but it keeps billing at the full rate for its provisioned tier and size.
Why it happens
Deleting a VM in Azure does not delete its disks unless you explicitly opt in. The OS disk, data disks, NICs, and public IPs are all separate resources with independent lifecycles, so they routinely outlive the VM.
Managed disks bill on provisioned capacity and tier, not on usage, so an idle premium SSD costs exactly the same as a busy one. Months later nobody is confident enough to delete disk-prod-04 because nobody remembers what it held.
What it costs / blast radius
A 1 TiB P30 premium SSD is roughly $135/month at list price, about $1,600/year, for a disk nothing is attached to. (List price; reserved or different tiers vary.) Multiply by the disks a few years of VM churn leaves behind and it stops being a rounding error.
See it
Resources
| where type =~ 'microsoft.compute/disks'
| where properties.diskState =~ 'Unattached'
| project name, resourceGroup, subscriptionId, location,
sku = sku.name, sizeGB = properties.diskSizeGB// Snapshot first if the data might matter, then remove the orphan.
az snapshot create --name disk-prod-04-snap \
--source disk-prod-04 --resource-group rg-prod
az disk delete --name disk-prod-04 --resource-group rg-prod --yesHow StratoLens helps
StratoLens inventories every managed disk across every subscription, flags the unattached ones with their per-disk monthly cost, and keeps the change history so you can see when the disk was orphaned and what it was last attached to. The "is it safe to delete" question gets an answer instead of a shrug.